Sintered and carburized porous stainless steel part and method thereof

ABSTRACT

This invention presents a sintered and carburized porous stainless steel part with improved strength and hardness and method thereof. The sintered and carburized porous stainless steel part has a porous body with a relative density between 30% and 89%, which is sintered from a stainless steel powder, wherein exposed pore surfaces inside the porous body are carburized without forming carbides and without using an activation process in advance. A carburized layer is formed and spread into the core of the sintered porous body. Thereby, the strength, surface hardness, and core hardness of the sintered body are significantly increased.

This application is a continuation-in-part, and claims priority, of from U.S. patent application Ser. No. 13/074,652 filed on Mar. 29, 2011, entitled “METHOD FOR ENHANCING STRENGTH AND HARDNESS OF POWDER METALLURGY STAINLESS STEEL”

FIELD OF THE INVENTION

The present invention relates to a porous stainless steel part, particularly to a sintered and carburized porous stainless steel part with improved strength and hardness and method thereof.

BACKGROUND OF THE INVENTION

Powder metallurgy has been extensively used to fabricate various metallic products. A various techniques had been disclosed, such as U.S. Pat. No. 6,669,898, U.S. Pat. No. 5,985,208, U.S. Pat. No. 7,211,125, U.S. Pat. No. 4,708,741, U.S. Pat. No. 7,311,875, U.S. Pat. No. 5,460,641, and U.S. Pat. No. 5,856,625. In the conventional powder metallurgy technology (press-and-sinter technology), the metal powders to be sintered are mixed uniformly beforehand with lubricants. Next, the mixture of the metal powders is compacted into a green compact. Next, the green compact is heated to a high temperature and sintered at that high temperature for a period of time. Thereby, interdiffusion of atoms in the particles occurs and forms a sintered body.

In addition to the abovementioned powder-compaction process, a Metal Injection Molding (MIM) process, which integrates the powder metallurgy process and the plastic injection molding process, is often used to fabricate structural parts having complicated shapes and superior mechanical properties. In the MIM process, metal powders are mixed with binders to form a feedstock, and an injection molding machine is used to inject the feedstock into a mold to form a green compact. The green compact is debinded and then sintered at a high temperature to obtain a sintered body.

The stainless steel materials fabricated in the abovementioned powder metallurgy (including press-and-sinter and MIM) process may be categorized into the high-density stainless steel materials and the porous low-density ones. Both types of these sintered stainless steels, particularly those with an austenitic structure like 316L, are usually soft, which limits the applications. Normally, the hardness and strength of stainless steels can be improved via a work-hardening method, such as rolling, forging, or cold heading. However, the abovementioned hardening method is unsuitable for a powder metallurgy sintered body because of its complicated shapes. Chromium plating and shot-peening processes have also been used to increase the surface hardness of stainless steels. However, the chromium coating is expensive and good adhesion to the substrate is difficult to achieve. Shot-peening can only increase the hardness at the outer surface and by a limited extent. For parts with complicated shapes, some regions are difficult to be shot-peened. Therefore, shot-peening is not an effective hardening method either for powder metallurgy parts. When powder metallurgy parts are porous, it becomes even more difficult to use the abovementioned processes to strengthen or harden the sintered body. In summary, there is few cost-effective method available to improve the strength and hardness of sintered porous stainless steel materials.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to increase the strength and hardness of sintered porous stainless steels.

To achieve the abovementioned objectives, the present invention proposes a sintered porous stainless steel part with high apparent hardness and strength, comprising a porous body with a relative density at a range between 30% and 89%, which is sintered from a stainless steel powder, wherein exposed pore surfaces inside the porous body are carburized without forming carbides and without the need to activate the part with halogenated materials in advance. With a hardened surface layer in all internal interconnected open pore channels, the overall/apparent strength and hardness are significantly increased.

To achieve the abovementioned objectives, the present invention further proposes a method for manufacturing sintered porous stainless steel parts, comprising steps of: fabricating a sintered stainless steel body, wherein the sintered stainless steel body has a porous body with a relative density at a range between 30% to 89%; and carburizing the sintered stainless steel body directly by a non-halogenated carbon-bearing gas, without any activation process in advance, wherein the sintered stainless steel body are held at a carburizing temperature below 600° C. such that carbon atoms can be implanted into exposed pore surfaces inside the porous body and form a carburized layer.

The sintered porous stainless steel parts and the method of the present invention can achieve the following goals:

1. For porous powder metallurgy stainless steels, the carburized layer spread into the core of the sintered body. Thereby, the surface hardness and core hardness of the sintered body are significantly increased.

2. In the present invention, the carburized layer is produced at a temperature below 600° C. without forming carbides and without carrying out any activation or cleaning process using halogenated materials in advance. Thus, chromium in the stainless steel will not react with carbon to form chromium carbide and thus the superior corrosion resistance can be retained. Therefore, the present invention can increase strength and hardness of porous powder metallurgy stainless steels while the superior corrosion resistance can still be preserved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical microscopic image of the microstructure of a sample used in Embodiment I.

FIG. 2 is an optical microscopic image of the microstructure of a sample used in Embodiment II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical contents of the present invention are described in detail in cooperation with the drawings.

The present invention proposes a sintered porous stainless steel part, comprising a porous body with a relative density at a range between 30% and 89% such that most pores in the porous body are interconnected open pores, wherein exposed pore surfaces of interconnected open pores inside the porous body are carburized without forming carbides and without activating/cleaning the surface of the sintered porous stainless steel part prior to carburization. In one embodiment of the present invention, the stainless steel powder is an iron-based material containing less than 1.0 wt % silicon, less than 2.0 wt % manganese, 8.0-19.0 wt % chromium, less than 15.0 wt % nickel, less than 6.0 wt % molybdenum, and less than 6.0 wt % copper.

The method for manufacturing sintered and carburized porous stainless steel parts according to one embodiment of the present invention comprises two main processes, sintering and carburizing. In the first step, the sintered stainless steel body is made from a stainless steel powder that is an iron-based material containing less than 1.0 wt % silicon, less than 2.0 wt % manganese, 8.0-19.0 wt % chromium, less than 15.0 wt % nickel, less than 6.0 wt % molybdenum, and less than 6.0 wt % copper. In one embodiment, the stainless steel powder is preferred to meet the chemical composition of 316L, 304L, or 17-4PH stainless steel regulated in the AISI (American Iron and Steel Institute) standard.

To obtain a sintered stainless steel body, the stainless steel powder is fabricated into a green compact via compacting stainless powder (press-and-sinter method) or molding stainless steel feedstock (MIM method). The green compact is placed in a non-oxidizing environment and maintained at a sintering temperature to sinter the green compact into a sintered body. Alternatively, the sintered stainless steel body may be sintered from loose powders which are placed in a mold without compaction. The non-oxidizing environment may be vacuum or hydrogen-bearing atmosphere. The hydrogen-bearing atmosphere is preferred to have greater than 5.0 vol % of hydrogen. The sintering temperature ranges from 1,050° C. to 1,400° C. The sintering process can be undertaken in an atmosphere sintering furnace or a vacuum sintering furnace. After the green compact is placed in the atmosphere sintering furnace, a gas mixture of hydrogen and nitrogen or cracked ammonia (N₂-75% H₂) is supplied to the furnace, and the atmosphere sintering furnace is heated to the sintering temperature and binders or lubricants in the green compact are removed during this heating period; the green compact is then maintained at the sintering temperature for a predetermined interval of time to form a sintered body. Next, the part is cooled to the ambient temperature and is taken out from the atmosphere sintering furnace. Alternatively, the green compact is placed in a vacuum furnace. The vacuum furnace is pumped to a given degree of vacuum, and the vacuum furnace is heated to the sintering temperature; the green compact is maintained at the temperature for a predetermined interval of time to form a sintered body. The predetermined interval of time ranges from 30 minutes to 3 hours. In the present invention, the sintering temperature or the sintering time is controlled to make the sintered body have a relative density between 30% and 89% so that a porous structure having interconnected open pores are obtained.

In the second processing step, the sintered body is directly carburized by a non-halogenated carbon-bearing gas at a temperature lower than 600° C., without an activation process in advance. In comparison with the conventional process, most low temperature carburization process requires an activation process or other cleaning processes to remove a passivated surface layer prior to carburization using halogenated materials, which is a not an environmentally-friendly process and is costly. In the present invention, as the sintered body is placed in a carbon-bearing atmosphere and maintained at a carburizing temperature, preferably between 400° C. and 580° C., carbon atoms are implanted into exposed pore surfaces and form a carburized layer inside the porous body. The abovementioned carburization process can be carried out under one atmosphere pressure or under a partial vacuum above 1 mbar.

In the present invention, the non-halogenated carbon-bearing gas is selected from a group consisted of carbon monoxide, methane, ethylene, acetylene, propane, and a mixture thereof The sintered body is maintained at the carburization temperature and carburized for a given interval of time; then the furnace is cooled to the ambient temperature. The carburization time is preferably set at 6-30 hours. When the sintered body has a relative density between 30% and 89%, most pores are interconnected open pores so that exposed pore surfaces can be carburized by the carbon-bearing gas. Thus, the carburized layer is spread all over the sintered body from exterior to interior. In the present invention, sintering might be undertaken in an atmosphere sintering furnace or vacuum furnace, and carburization might be undertaken in a carburizing furnace. Alternatively, both sintering and carburization may be undertaken in the same furnace. For example, after sintering is completed, the sintered body is not taken out from the atmosphere sintering furnace or vacuum sintering furnace, and a carbon-bearing atmosphere is directly supplied to the same furnace to undertake carburization.

Below, embodiments are used to demonstrate the sintered-and-carburized porous stainless steel parts and the method of the present invention. However, the embodiments are only to exemplify the present invention but not to limit the scope of the present invention. Table.1 lists the chemical compositions of the stainless steel powder used in the embodiments and comparisons. Composition 1 meets the specification of commercial 316L stainless steel powder. This “Composition 1” powder has an average particle diameter of 12.1 μm. Composition 2 also meets the specification of commercial 316L stainless steel powder. This “Composition 2” powder has an average particle diameter of 39.7 μm. “Composition 3” powder is a commercial 304L stainless steel powder having an average particle diameter of 40.2 μm. Table.2 lists the compositions and fabrication conditions of the samples used in Embodiments I-V. Table.3 lists the compositions and fabrication conditions of the samples used in Comparisons I-V. Table.4 lists the properties of the samples used in Embodiments I-V. Table.5 lists the properties of the samples used in Comparisons I-V.

In preparing the samples of the embodiments and comparisons, a stainless steel powder is mixed with a specified proportion of a lubricant or a specified proportion of a binder, and the mixture is fabricated into a green compact with a powder compaction process or an MIM process. Next, the green compact is debinded to remove the lubricant and the binder, and followed by sintering. Alternatively, a stainless steel powder is directly placed in a mold and then sintered, which is usually called loose-powder-sintering. Next, the sintered samples are carburized. The sintering and carburization parameters are listed in Tables 2 and 3. Then the density, hardness, strength, and corrosion resistance of sintered and sintered-and-carburized material and the thickness of the carburized layer are measured. In the embodiments and comparisons, the MIM process or the powder compaction process or loose-powder-sintering process are used to exemplify the fabrication process. However, the present invention does not exclude those with other powder metallurgy processes. The density of a sintered body is obtained with the Archimedes method. The density and the theoretical density are used to work out the relative density of the sintered body.

The mechanical property tests include the surface hardness and core hardness tests. The microhardness is measured using a Vickers hardness tester, which has a microscope, while the apparent (bulk, macroscopic) hardness is measured using a Rockwell hardness tester. The tensile strength and elongation are also measured. The corrosion resistance is tested following the MPIF (Metal Powder Industries Federation) Standard 62 and the frequently-used salt-spray test method. In the MPIF Standard 62, the workpiece is immersed in a 2 wt % sulfuric acid solution for 24 hours. Then, the weight loss is measured. If the weight loss per square decimeter is less than 0.005 g, the workpiece is a qualified one and designated by O. If the weight loss per square decimeter is greater than 0.005 g, the workpiece is an unqualified one and designated by X. The workpieces are also tested with the salt-spray method, wherein the workpieces are placed in a mist of 5 wt % sodium chloride solution and the surface is examined with naked eyes to determine the interval of time before corrosion occurs. The thickness of the carburized region is measured via observing the microscopic images of the carburized workpiece. The mechanical properties and corrosion resistances of Embodiments I-V are listed in Table.4.

Embodiment I

This embodiment adopts a stainless steel powder of Composition 2 and a mean particle size of 39.7 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and then sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body 10 a. After being cooled, the sintered body 10 a is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. The sintered body 10 a has a relative density of 86% and has a microstructure shown in FIG. 1. As can be observed in FIG. 1, the sintered and carburized body 10 a has a carburized region 11 a (the white region). The sintered body 10 a has a macroscopic (apparent) hardness of HRB75, a surface microhardness of HV820 and a core microhardness of HV220. The sintered body 10 a has a tensile strength of 520 MPa and an elongation of 20%. The sintered body 10 a has qualified corrosion resistance and can tolerate the salt spray test for 6 hours. This embodiment demonstrates that, when sintered body has a low relative density, pores are interconnected and are connected to the outer surface, and the carburization will occur inside the sintered body. Therefore, the overall strength and hardness of the sintered body can be improved after carburization.

Embodiment II

This embodiment adopts a stainless steel powder of Composition 3 and a mean particle size of 40.2 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and then sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body 10 b. After being cooled, the sintered body 10 b is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body 10 b has a relative density of 86% and has a microstructure shown in FIG. 2. It can be observed in FIG. 2 that the sintered and carburized body 10 b has a carburized region 11 b (the white region). The sintered body 10 b has a macroscopic (bulk) hardness of HRB74, a surface microhardness of HV811 and a core microhardness of HV245. The sintered body 10 b has a tensile strength of 519 MPa and an elongation of 16%. The sintered body 10 b has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment III

This embodiment adopts a stainless steel powder of Composition 1 and a mean particle size of 12.1 μm. The stainless steel powder is processed with the MIM process to form a green compact. The green compact is debinded and then sintered in an atmosphere furnace under cracked ammonia at a sintering temperature of 1,200° C. for 1 hours to form a sintered body. After being cooled, the sintered body is taken out from the furnace and placed in a low pressure carburizing (vacuum carburizing) furnace for carburization at a temperature of 480° C. for 12 hours using a mixture of acetylene, ethylene, and hydrogen at a pressure of 10 mbar. The sintered body has a relative density of 76% and has a carburized region. The sintered body has a surface microhardness of HV801, a core microhardness of HV242, and a high macroscopic hardness of HRB95, which is attributed to the use of cracked ammonia atmosphere.

Embodiment IV

This embodiment adopts a stainless steel powder of Composition 2 and a mean particle size of 39.7 μm. The stainless steel powder is processed with loose-powder-sintering. The powder is sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. The sintered body has a relative density of 50% and has a carburized region inside the porous body. The sintered body has a hardness of HRH18, a surface microhardness of HV815 and a core microhardness of HV488. The core microhardness of Embodiment IV is higher than that in the Embodiment III because of its higher amounts of pores, which facilitate the penetration of the carburizing gas. However, the macroscopic hardness is softer than those of Embodiments 1, 2, and 3 due to its lower sintered density.

Embodiment V

This embodiment adopts a stainless steel powder of Composition 3 and a mean particle size of 40.2 μm. The stainless steel powder is processed with loose-powder-sintering. The powder is sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 50% and has a carburized region in the core. The sintered body has a hardness of HRH16, a surface microhardness of HV818 and a core microhardness of HV482.

Comparison I

This comparison adopts the same stainless steel powder and sintering process as those used in Embodiment I. The sintered porous body has a relative density of 86%, a macroscopic hardness of HRB25, a surface microhardness of HV132, and a core microhardness of HV135. The sintered body has a tensile strength of 295MPa and an elongation of 24%. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Comparison II

This comparison adopts the same stainless steel powder and sintering process as those used in Embodiment II. The sintered porous body has a relative density of 86%, a macroscopic hardness of HRB27, a surface microhardness of HV135, and a core microhardness of HV138. The sintered body has a tensile strength of 291 MPa and an elongation of 25%. The sintered body has qualified corrosion resistance and can pass the salt spray test of 6 hours. The strength and hardness of the sample of this comparison are lower than those of the sample of Embodiment II.

Comparison III

This comparison adopts the same stainless steel powder and sintering process as those used in Embodiment III. The sintered body has a relative density of 76%, a macroscopic hardness of HRB16, a surface microhardness of HV121, and a core microhardness of HV122.

Comparison VI

This comparison adopts the same stainless steel powder and sintering process as those used in Embodiment IV The sintered porous body has a relative density of 50%, a surface microhardness of HV110, and a core microhardness of HV115. The macroscopic hardness HRH of the sintered body is too low to be measured in this comparison.

Comparison V

This comparison adopts the same stainless steel powder and sintering process as those used in Embodiment V. The sintered body has a relative density of 50%, a surface microhardness of HV 112, and a core microhardness of HV113. The macroscopic hardness HRH of the sintered body is too low to be measured in this comparison.

In Embodiments I-V, when the sintered bodies have porous microstructures, carbon-bearing gas can permeate into the core of the sintered body through interconnected open pore channels and carburize exposed pore surfaces inside the sintered body. Thereby, not only the hardness at the outer surface is increased, the core hardness and the overall strength are also increased. For example, the surface microhardness of the sintered body in Embodiment V is increased to HV818, and the core microhardness is increased to HV482. Further, the tensile strengths of the sintered bodies are also significantly increased compared to those of the Comparisons I and II while the corrosion resistance of stainless steel is still preserved.

In conclusion, the present invention implants carbon atoms into a porous sintered body to form a carburized layer in the sintered body, whereby the hardness and strength of the porous sintered body is increased by the hard carburized layer at exposed pore surfaces When the sintered body is a dense compact, carbon atoms diffuse only into the outer surface of the sintered body to form the carburized layer in the surface, but not in the core of the sintered body. Compared to the conventional chromium-plating method and shot-peening method, the present invention improves the overall strength and hardness of powder metallurgy porous stainless steels more effectively. As the carburized layer is formed at a temperature of lower than 600° C., chromium would not react with carbon to form chromium carbide. Therefore, the present invention can increase the strength and hardness of powder metallurgy porous stainless steels and with the superior corrosion resistance retained.

The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention. 

What is claimed is:
 1. A sintered porous stainless steel part with high apparent hardness and strength, comprising a porous body and with a relative density at a range between 30% and 89%, which is sintered from a stainless steel powder, wherein exposed pore surfaces inside the porous body are carburized without forming carbides and without using any activation process with halogenated materials in advance.
 2. The sintered porous stainless steel part according to claim 1, wherein the stainless steel powder is an iron-based material containing less than 1.0 wt % silicon, less than 2.0 wt % manganese, 8.0-19.0 wt % chromium, less than 15.0 wt % nickel, less than 6.0 wt % molybdenum, and less than 6.0 wt % copper.
 3. A method for manufacturing sintered and carburized porous stainless steel parts, comprising steps of: fabricating a sintered stainless steel body, wherein the sintered stainless steel body comprises a porous body and has a relative density at a range between 30% to 89%; and carburizing the sintered porous stainless steel body directly by a non-halogenated carbon-bearing gas, without any activation process in advance, wherein the sintered stainless steel body being maintained at a carburizing temperature below 600° C. such that carbon atoms can be implanted into exposed pore surfaces and form carburized layers inside the porous body.
 4. The method for manufacturing sintered porous stainless steel parts according to claim 3, wherein the non-halogenated carbon-bearing gas is selected from a group consisted of carbon monoxide, methane, ethylene, acetylene, propane and a mixture thereof.
 5. The method for manufacturing sintered porous stainless steel parts according to claim 3, wherein the sintered stainless steel body is sintered under a vacuum or a hydrogen-bearing atmosphere.
 6. The method for manufacturing sintered porous stainless steel parts according to claim 3, wherein the carburizing temperature ranges from 400° C. to 580° C.
 7. The method for manufacturing sintered porous stainless steel parts according to claim 3, wherein the sintered stainless steel body is sintered from a green compact that is fabricated with a Metal Injection Molding method.
 8. The method for manufacturing sintered porous stainless steel parts according to claim 3, wherein the sintered stainless steel body is sintered from a green compact that is fabricated with a powder-compaction method.
 9. The method for manufacturing sintered porous stainless steel parts according to claim 3, wherein the sintered stainless steel body is sintered from loose powders which are placed in a mold without compaction.
 10. The method for manufacturing sintered and carburized porous stainless steel parts according to claim 3, wherein the sintered stainless steel body is sintered from a green compact of a stainless steel powder, which is an iron-based material containing less than 1.0 wt % silicon, less than 2.0 wt % manganese, 8.0-19.0 wt % chromium, less than 15.0 wt % nickel, less than 6.0 wt % molybdenum, and less than 6.0 wt % copper. 